Can you use a vacuum pump on a charged A/C System?

Question

I have a vehicle that is not currently running from which I am pulling the engine. To do so, I need to remove the A/C compressor and condensor, but it is currently charged.

I have borrowed an A/C manifold gauge set and an A/C vacuum pump from AutoZone. I thought I could use this not just to put a vacuum on the system later when I swap to a different vehicle, but to evacuate the R134a refrigerant currently in the system. However, the instructions for the vacuum pump say not to run it on a pressurized system.

So what do I do? I can't take the car to a mechanic to have them properly pull out the refrigerant to a recovery system because the car is not mobile. I know you're not supposed to just dump R134a to atmosphere (though it's not as bad as R12).

Can I run the vacuum pump on the system anyway? Is that effectively the same as dumping it to atmosphere? Where do I go from here?

Thanks!

Answer 1

I don't need your end goal here, but in almost every vehicle I have worked on I have never had to remove the A/C compressor with the engine. They are designed to be disconnected from the engine and put to the side so you don't have to discharge the refrigerant from it.

If you absolutely believe you need to remove or discharge the system, take it to a shop which can draw out the r134a from the system in an environmentally safe way. You should be able to work out some kind of deal with them to bring the vehicle back to them for recharge after it's done. Remember, though, you will have to replace the dryer when you put it back together, as well as recharge the r134a.

Using the vacuum pump to remove the r134a will do the same thing as just dumping it directly into the atmosphere. If you cannot move the vehicle and only want to dump it, you really need to be careful of the r134a freeze burning you as it decompressed out of the system. Obviously, I don't recommend this approach if at all possible.

Answer 2

There's absolutely no need to use a vacuum pump to remove the refrigerant. It's really easy to remove the refrigerant by condensing it out.

If you're a shade tree mechanic, this is what has worked for me to recover and store R134a:

1. Get a refrigerant recovery tank with 1/4" fittings.

   You could adapt an empty propane tank by using up all the propane, venting the remainder to atmosphere, removing the original valve, purging with water, then screwing in adapters, a valve, and a 1/4" fitting. Propane tanks are rated at 375PSI, while R134a at 100F is ~150PSI so as long as you don't let the tank get too hot, you won't overload it.

2. Get an automotive HVAC gauge/manifold with high/low/vacuum valves, hoses and high/low side R134a couplers.

3. Get an automotive vacuum pump (two-stage).

4. Attach the low side blue hose and coupler to the manifold, as well as the yellow service hose to the tank, and black vacuum hose to the pump. Open the tank valve.

5. Open the VACuum and LOW valves. Close the LOW R134a coupler. Evacuate the system - i.e. tank and three hoses. When done, close the VAC valve, disconnect the pump and black hose from the manifold. At this point the tank, the manifold, the yellow service and blue low side hoses are evacuated.

6. Locate a chest freezer near to the car (e.g. in the garage). Put the tank inside the chest freezer. Let it freeze overnight.

7. Ensure the car has cooled down overnight. Do not turn the car on.

8. Close the LOW valve. Attach the LOW-side coupler to the car. Open the coupler. Slowly open the LOW valve - ensure that the pressure doesn't exceed the expected R134a pressure at ambient temperature (150psi at most). This drains the refrigerant into the tank via the low-side port. The refrigerant will be condensing in the tank. At 0F (typical freezer temperature), the remaining pressure will be around 6PSI.

9. Put the tank in a plastic bucket, fill with dry ice. Put back into the freezer as this slows down the sublimation. You've now lowered the tank's vapor phase pressure to well below 22"Hg.

   When done, assuming that there was no air in the car's system, the pressure should be still below zero as almost all of the refrigerant vapor will be condensed in the tank, with very little left. You should strive for vapor pressure of 20"Hg vacuum or less.

10. Close all valves, including tank valve. You're done removing R134a from the system.